This is not how it works. AllBasically all three files are taken into consideration.
WordPress/your server will check in the following order:
wp-config.php <> .htaccess <> php.ini
You can´t "overwrite" what is definedIf something on a higher level, so in your case the"higher level" .htaccess would be(later in the chain) is limiting factor.
Update: I was not really clear in my expressionyour value, let me improve itthe former value will be ignored or overwritten.
What I meant If a piece of the chain is missing, that you can not simply define athe value you like in wp-config.php and .htaccess or php.ini are then completely ignoredfor example, the next higher one will be used instead.
ActuallyIn your example the value from wp-config.php can be "overwritten" by a lower value in .htaccess orwould limit the php.inimax_upload_size to 64 MB, so depending onalthough your server configuration, your php.ini will set a limit you can´t breakwould be fine with up to 256 MB and WordPress would also accept up to 128 MB in file size first.
You can try toalso set a different limit via the .htaccess file and by doing this will workyou overwrite the value from php.ini, so that it is no longer the dominating value. This works in a lot of hosting environments as the servers, so chances are configured thatquite good you can raise or lower the max_upload_size in this way.
If you have a more strictly configured hosting/server, chances areit is possible that the option to override the settings from php.ini is disabled. IfIn this is the case, a pre-definedoverriding max_upload_size can´t be changed and you have tofrom .htaccess won´t work with what you got, so this could be a limiting factor.